1. Field of the Invention
This invention relates to ferroelectric nonvolatile memory and an oxide multi-layered structure.
2. Related Art
Studies on materials of oxide thin films started from high temperature superconducting oxide ((1) Z. Phys. B., 64, 189-193 (1986)), and have been developed dramatically in a few years.
Memory devices using ferroelectric materials were extensively studied once in the 1950s but did not spread into the industry because of the difficulty of interface control of ferroelectric thin films. Recently, however, ferroelectric nonvolatile memory (for example, (2) Electrical Engineering, 71, 916-922 (1952), (3) Bell Labs. Record, 33, 335-342 (1955)) has attracted attention and has been under rapid development (for example, (4) Appl. Phys. Lett., 48, 1439-1440 (1986), (5) U.S. Pat. No. 4,713,157, (6) IEDM Tech. Dig., 850-851 (1987), (7) IEEE J. Solid State Circuits, 23, 1171-1175 (1988), (8) Tech. Dig. ISSCC 88, 130-131 (1988)). The present status of ferroelectric nonvolatile memory is reported in detail (for example, (9) Oyo Buturi (Applied Physics), 62, No. 12, 1212-1215 (1993), (10) Electronic Ceramics, 24, No. 7, 6-10 (1993), (11) Denshi Zairyo (Electronic Parts and Materials), 33, No. 8, Application for nonvolatile memory by ferroelectric thin films, special edition (1994), (12) Ceramics Japan, 27, 720-727 (1992)).
Many of ferroelectric nonvolatile memory devices heretofore reported have a structure in which a ferroelectric oxide thin film having a perovskite crystal structure is interposed between a pair of Platinum (Pt) electrodes (for example, (13) J. Appl. Phys., 70, 382-388 (1991)). In typical ferroelectric nonvolatile memory, a PZT thin film is made between a pair of Pt electrodes. However, Pt electrodes are liable to peel off, and the fatigue property indicative of the aging of the memory is bad in most cases. It is presumed that these problems are caused by complex mixture of various factors, such as oxygen defects in the PZT thin film near the interface with the Pt electrode, and a fatigue in coupling force due to a large spontaneous polarization value, i.e. a large displacement in the lattice (for example, (14) J. Appl. Phys., 70, 382-388 (1991)).
In order to improve the fatigue property of ferroelectric nonvolatile memory, the use of a bismuth (Bi) layered ferroelectric oxide thin film as ferroelectric thin film has been proposed (for example, (15) PCT International Publication W093/12538, (16) PCT International Publication W093/12542)).
However, according to the knowledge of the Inventor, ferroelectric nonvolatile memory heretofore proposed is not satisfactory in optimization of the structure.